There are three stages in the development of compounds. At first the components of the compound expression are written separately; next they are united by a hyphen; finally, when the separate significance and accent of these components have been lost sight of, they are combined into one word. The hyphenated stage may thus be considered merely preparatory to the coalescence of the various members into one word. Many such compounds have now fully coalesced and are written as one word (e.g. footwall, landmass, offshore, rockburst).

Words used in their ordinary grammatical relationship — for example, noun and attributive adjective — ought not to be hyphened. A typical example of this rule is afforded by adverbs ending in ly standing before the words they modify, for example thinly banded rocks. The relationship in this case is clear, and the hyphen is omitted. To show that the syntactical relationship between two words is close, use the hyphen.

Whenever the compound expression has a meaning different from that borne by its components in their ordinary grammatical relationship, the hyphen is used, as in the expression bird's-eye (referring to a spot, blob, or irregular patch of sparry calcite commonly found in limestones and some dolomite, as a precipitate that infills cavities). See 'Hyphen' in 'Punctuation' section for details.